The high strength steel plate of a tensile strength of 780 MPa or more used as members for construction machines, industrial machinery, bridges, buildings, ships, and other welded structures is now being required to provide both high strength and high toughness of the base material, satisfy the requirements of high weldability by a preheat-free process, and be able to be produced inexpensively in a short time in plate thicknesses of 40 mm or so. That is, it is necessary to satisfy the requirements of high strength and high toughness of the base material and a preheat-free process at the time of shielded arc, TIG, and MIG welding or other small heat input welding by an inexpensive system of ingredients, a short work time, and an inexpensive production process.
As the conventional method of production of high strength thick-gauge steel plate of a tensile strength of 780 MPa or more giving high weldability, for example, as disclosed in PLTs 1 to 3, there is the method of rolling the steel plate, then immediately directly quenching it on-line, then tempering it, that is, using direct quenching and tempering.
Further, for a non-heat treatment type of method of production of high strength thick-gauge steel plate of a tensile strength of 780 MPa or more not requiring reheat tempering heat treatment after rolling, for example, there are the disclosures in PLTs 4 to 8. Each is a method of production superior in production period and productivity in the point that the reheat tempering heat treatment can be omitted. Among these, the inventions described in PLTs 4 to 7 relate to a method of production by an accelerated cooling-interim stop process comprising rolling steel plate, then accelerated cooling it, then stopping midway. Further, the invention described in PLT 8 relates to a method of production of rolling, then air cooling down to room temperature.
Citation List
Patent Literature
PLT 1: Japanese Patent Publication (A) No. 03-232923
PLT 2: Japanese Patent Publication (A) No. 09-263828
PLT 3: Japanese Patent Publication (A) No. 2000-160281
PLT 4: Japanese Patent Publication (A) No. 2000-319726
PLT 5: Japanese Patent Publication (A) No. 2005
PLT 6: Japanese Patent Publication (A) No. 2004-52063
PLT 7: Japanese Patent Publication (A) No. 2001-226740
PLT 8: Japanese Patent Publication (A) No. 08-188823